1. Field of the Invention
The present invention relates to an objective lens system with a high speed of about F1.2 and a field angle of about 46 degrees, and more particularly to such an objective lens system with an aspheric surface.
2. Description of the Prior Art
It is generally known in the prior art that a Gauss-type lens system with a high speed of about F1.2 and a sufficiently long back focal distance achieves such a desirable correction for field curvature that the resolution of the image is satisfactory throughout the entire area. In such a Gauss-type lens system, however, the spherical aberration is generally in an overcorrection condition to remarkably deteriorate the contrast of the image due to the greatly generated sagittal flare. The overcorrection of the spherical aberration is caused by the steep curvatures of the negative lens elements sandwiching the aperture stop, and is hardly corrected even by utilizing a glass material of a high refractive index in the case of such a high speed lens system.
In this field of art, it has been known to utilize an aspheric surface for aberration correction.
Examples of the prior art which discloses a Gauss-type objective lens system with an aspheric surface can be found in U.S. Pat. No. 3,459,468, Japanese Patent Publication No. 47-19386 and Japanese laid open Patent Application No. 56-75613.
The prior art is still seeking to provide a high speed Gauss-type objective lens system with an improved optical performance.